Generally, an organic electroluminescent device has a laminated structure comprising a transparent electrode layer, a metallic electrode layer, and an organic interlayer including an organic luminescent layer arranged between the two electrodes. In order to increase the luminance efficiency, the organic interlayer may further include hole transport agents and electron transport agents, often in a multilayer configuration.
In case of a direct current(DC) operation, the transparent electrode functions as an anode, and the metallic electrode as a cathode. On applying a voltage to the organic electroluminescent device, the organic luminescent layer emits light towards the transparent electrode layer, wherein the metallic electrode layer functions as a metal mirror which reflects the light originated from the organic luminescent layer. However, as the color of most metallic electrode layers is silver-white, it is often difficult to discern the light-emitting area when the device is placed in a bright environment.
In order to solve the contrast problem in such devices as cathode ray tube, liquid crystal device(LCD) and inorganic electroluminescent device(inorganic ELD), there have been employed two methods, a front attachment method and a rear insert method.
The front attachment method improves the contrast ratio of a cathode ray tube or LCD by applying a circular polarizer, or a color filter, to the front side of a luminescent layer(see, e.g., U.S. Pat. No. 5,596,246; U.S. Pat. No. 5,523,114; U.S. Pat. No. 4,351,681; and U.S. Pat. No. 3,682,531). This method, however, requires a complicated process entailing a high manufacturing cost.
In the rear insert method, a black metallic electrode layer is arranged behind a luminescent layer for enhanced contrast. Alternatively, a light-absorbing layer may be inserted either between a luminescent layer and a metallic electrode layer as an insulating layer, or between an insulating layer and a metallic electrode layer. For the formation of a black metallic electrode layer, U.S. Pat. No. 4,859,904 describes the use of such materials as transition metal oxides, transition metal sulfides, earth metal oxides and earth metal sulfides, e.g., PbS, PbO, CuO, MnO.sub.2, Tb.sub.4 O.sub.7, Eu.sub.2 O.sub.3, PrO.sub.2 and Ce.sub.2 S.sub.3. Further, U.S. Pat. No. 4,963,788, U.S. Pat. No. 4,870,322, U.S. Pat. No. 4,758,765, U.S. Pat. No. 4,634,639, U.S. Pat. No. 4,455,506, and U.S. Pat. No. 4,287,449 disclose the use of darkly-dyed fillers, metal-doped zinc sulfides, chromes or aluminum oxides as constituents of the light-absorbing layer.
However, the above rear insert method suffers from the problem that the insert layer leads to the generation of an electronic band structure different from that of the original organic electroluminescent device comprising organic double layers, e.g., a hole transport layer and a luminescent/electron transport layer.
Accordingly, there has existed a need to develop a new method for increasing the contrast ratio of an organic electroluminescent device, without the problems of the prior art.